Liposomes are self-closed colloidal particles in which membranes composed of one or more lipid bilayer(s) encapsulate a fraction of the aqueous solution in which they are suspended. The surfaces of bilayers are hydrophilic while the interior of bilayers, which contain hydrocarbon chains, are hydrophobic. Because of the different microenvironments in their structure, liposomes can encapsulate hydrophilic molecules, bind molecules on the bilayer surfaces or dissolve hydrophobic molecules into the middle of the bilayer. Their ability to incorporate many types of molecules has resulted in applications for drug delivery, diagnostics, cosmetics, cosmeceuticals and nutraceuticals.
Liposomes are manufactured by several different methods. Typically, the process begins when a lipid or combination of lipids are dissolved in an organic solvent. Upon removal of organic solvents and hydration, large multilamellar vesicles (MLVs) are formed. For some applications, small unilamellar vesicles (SUVs) may be desired. SUVs can be produced from MLVs by several techniques including sonication, extrusion through membranes with well-defined pores, French press extrusion and homogenization.
Problems associated with liposomes include colloidal instability, difficulty in scale-up sterilization, and variability between batches in manufacturing. Liposome preparation and manufacturing typically involves removal of organic solvents followed by extrusion or homogenization. These processes may expose liposomal components to extreme conditions such as elevated pressures, elevated temperatures and high shear conditions which can degrade lipids and other molecules incorporated into the liposomes.
Liposome preparations are often characterized by very heterogeneous distributions of sizes and number of bilayers. Conditions optimized on a small scale normally do not scale up well and preparation of large-scale batches is cumbersome and labor intensive.
Another issue associated with liposomes for medical uses is sterilization. Among heat sterilization, ethanol oxide exposure, gamma irradiation and sterile filtration, only the last technique is suitable for liposomes and then only for liposomes smaller than about 100 nanometers (nm). Filtration of liposomes poses many difficulties.
Another problem for liposome applications is colloidal stability. Liposomes in suspension can aggregate and fuse upon storage, heating and addition of various additives. Because of these stability problems, liposomes are often lyophilized. Lyophilization is costly and time consuming. Upon reconstitution, size distributions often increase and encapsulated materials may leak out from the liposomes.
It is therefore desirable to develop new methods and materials which address these problems with current liposome formulations.